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Windchill Temperature Index

In the United States and Canada, a measure of apparent temperature. When it's cold, a good wind makes it feel colder. The windchill temperature index attempts to quantify this sensation, describing it as the temperature with no wind at which one would feel equally cold. The current version uses the formula:

W = 35.74 + 0.6215 T − 35.75(V^0.16) + 0.4275 T (V^0.16)

where W is the windchill in degrees Fahrenheit, T is the temperature in degrees Fahrenheit, and V is the wind velocity at a height of 5 feet in miles per hour. The index is undefined at wind speeds below 3 miles per hour, and at temperatures above 50 degrees Fahrenheit. In SI units (degrees Celsius, kilometers per hour), the formula is:

W = 13.12 + 0.6215 T − 11.37(V^0.16) + 0.2965 T (V^0.16)

Chance of frostbite

Adapted from a National Weather Service chart.

The windchill index does not take sunlight into account. Bright sunlight can make one feel as much as 12 points warmer.

The windchill temperature index makes use of the temperature scales, but it is not really a measure of temperature. To signal that they are not temperatures, values are written without the degree symbol and spoken without the word “degree.” For example, “-5F”, not “-°F”; and “3 Celsius”, not “3 degrees Celsius”.

The current version of the index was introduced in November 2001 by the U.S. National Weather Service and Meteorological Services of Canada. It replaced an earlier index which tended to overestimate the effect of the wind at certain velocities. See www.nws.noaa.gov/om/winter/windchill-images/wind_chill_compare.pdf and the Canadian site referenced below.

resources

A printable version of the time-to-frostbite chart may downloaded from NOAA. You can choose a color pdf, a black and white chart, or even one in knots:

www.weather.gov/safety/cold-wind-chill-chart

Environment Canada maintains an excellent webpage on windchill. Access it through

https://ec.gc.ca/meteo-weather/default.asp?lang=En&n=5FBF816A-1